This invention relates generally to field emission display devices, and in particular, to methods of manufacturing cathodes for field emission devices.
Field emission displays (FEDs) are flat panel display devices that combine the size and portability advantages of liquid crystal displays (LCDs) with the performance of conventional cathode ray tubes (CRTs). FED devices typically include a field emission cathode positioned opposite a flat screen coated with phosphors. The phosphors emit light in response to bombardment by electrons from the cathode to produce an image. The field emission cathode emits electrons when subjected to an electric field of sufficient strength. The cathode typically includes thousands of microscopic emitter tips for each pixel of the screen. It is principally the emissive nature of the cathode that give FEDs the thin, flat screen features of an LCD with the viewing angle, brightness, and response speed of a CRT.
While FEDs are potentially very attractive devices, a limiting factor in the widespread adoption of the technology is the difficulty of manufacturing the devices, particularly the difficulty in manufacturing the FED cathodes. Field emission cathodes have been known for some time. See, for example, Spindt et al. J. of Appl. Phys. 47,5248 (1976). The field emission cathodes described therein typically comprise sharp-tip metal electron emitters, such as molybdenum cones having a tip radius of the order of a few tens of nanometers. A method of manufacturing such cathodes with Mo cone emitters on a conductive substrate using semiconductor fabrication techniques is described, of example, in U.S. Pat. No. 5,332,627 to Watanabe et al. Another example of the use of semiconductor fabrication techniques, including patterning and etching, to manufacture emitter cone structures is provided in U.S. Pat. No. 5,755,944 to Haven et al.
The benefits of using carbon in the form of graphite or diamond as the emitting material in a field emission cathode have been recognized. A manufacturing process that includes in situ growth of diamond emitter bodies, by for example, chemical vapor deposition (CVD) or flame deposition, or alternatively deposition of pre-existing diamond grit or powder is described in U.S. Pat. No. 5,747,918 to Eom et al. Another approach to fabricating a carbon-based field emitter is given in U.S. Pat. No. 5,608,283 to Twichell et al. which avoids diamond CVD and uses fewer semiconductor processing steps than some of the approaches reported above.
Despite the variety of processes for producing field emission cathodes that have been developed, there remains a need for improved manufacturing techniques that avoid the complications of previous approaches described above. It would be desirable for the improved techniques for field emission cathodes to be scalable so that large field emission displays can be fabricated at reasonable cost without defects.
Electrophoretic deposition provides an efficient process for manufacturing a field emission cathode. Particles of an electron emitting material are deposited by electrophoretic deposition on a conducting layer overlying an insulating layer to produce the cathode. According to an aspect of the present invention, insulating particles are mixed with electron emitting particles in the deposited layer. Desired properties of a field emission cathode include requisite adhesion strength of the emitting particles to the conducting layer, sufficient emission when an electric field is applied to the cathode, and spatial and temporal stability of the field emission. According to another aspect of the present invention, by controlling the composition of the deposition bath and by mixing insulating particles with emitting particles, an electrophoretic deposition process can be used to efficiently produce field emission cathodes with the desired characteristics.
Electron emitting materials that can be used for the emitting particles include metals, semiconductors, metal-semiconductor compounds, and forms of carbon. For example, graphite carbon, diamond, amorphous carbon, molybdenum, tin, and silicon, all in powder form, are advantageously used as emitting particles. Beneficial particle sizes are between about 0.05 xcexcm and about 20 xcexcm. Dispersed, rather than uniform, particle size distributions are preferred to improve packing.
The insulating particles are composed of a material that has a band gap that is greater than or equal to about 2 eV and is available in powder form. Particular examples of insulating materials used for the insulating particles include xcex3-alumina, other alumina phases, silicon carbide, and oxides of titanium and zirconium. Best results are achieved for insulating particles between about a quarter and about a half the characteristic size of the emitting particles. The ratio of emitting particles to insulating particles varies between about 0.1% to about 99% emitting particles by weight, preferably between about 5% and about 50% emitting particles, depending on the particular materials. For graphite carbon particles as emitting particles and xcex3-alumina particles as insulating particles, a mixture with about 20% graphite carbon particles by weight gives advantageous results.
In electrophoretic deposition, particles suspended in a deposition bath are deposited onto a conducting substrate under the influence of an electric field. The composition of the deposition bath plays a crucial role in the electrophoretic deposition process. According to an aspect of the invention, the deposition bath for the field emission cathode includes an alcohol, a charging salt, water, and a dispersant. The dominant component of the deposition bath is a reasonably hydrophilic alcohol such as a propanol, butanol, or an octanol. A charging salt such as Mg(NO3)2, La(NO3)2, or Y(NO3)2, at a concentration of between about 10xe2x88x925 to 10xe2x88x921 moles/liter is added to the alcohol. The metal nitrates partially dissociate in the alcohol and the positive dissociation product adsorbs onto the emitting particles and insulating particles charging them positively. The water content has a significant effect on the adhesion of particles to the conductive layer and to each other. The dissolved charging salt reacts with hydroxide ions from the reduction of water to form a hydroxide that serves as a binder. Water content of between about 1% and about 30% by volume is used to increase the adhesion of deposited particles. The deposition bath also includes a dispersant, for example, glycerin, at a concentration of from 1% to 20% by volume of the deposition bath. Particularly advantageous results are obtained for deposition of graphite carbon particles in the size range between about 0.1 and 1.0 xcexcm mixed with about 0.05 xcexcm xcex3-alumina particles in a ratio of 20:80 by weight in a deposition bath of isopropyl alcohol containing 10xe2x88x923 molar Mg(NO3)2 with 3% water by volume and 1% glycerin by volume.
The field emission cathodes produced according to the method of the present invention exhibit emission with excellent spatial and temporal stability. The emitting layer is a uniform deposit and has good adhesion to the underlying substrate. The field emission cathodes so produced can be used as an electron source in a field emission display device.